The present invention relates to a process for manufacturing grain-oriented silicon steel sheet having a high magnetic flux density, low core loss, and excellent magnetic properties. More specifically, it relates to the step therein of decarburization and primary recrystallization annealing prior to the secondary recrystallization annealing.
As is commonly known, grain-oriented silicon steel sheet is used primarily as the iron core in transformers and other electrical devices. This grain-oriented silicon steel sheet must have outstanding magnetic properties. This means that it must have a high magnetic flux density B.sub.10 value (magnetic flux density at a magnetizing force of 1000 A/m) and a core loss W.sub.17/50 value (core loss at a frequency of 50 Hz and a maximum flux density of 1.7 T).
The magnetic properties of such grain-oriented silicon steel sheet may be raised by achieving a high level of orientation in the secondary recrystallization &lt;001&gt; axis of the steel sheet or by restricting the amount of impurities and precipitates in the final product to an absolute minimum. A basic manufacturing process that achieves this by means of the two-stage cold rolling of grain-oriented silicon steel sheet was proposed by N. P. Goss, and has been upgraded by numerous modifications, which have produced constant improvements in magnetic flux density and core loss. Typical of these improvements are Japanese Patent Publication (Kokoku) No. 15644/1965, which proposes the utilization of an AlN precipitation phase, and Kokoku No. 14737/1982, which proposes the use of small amounts of molybdenum and antimony, and trace quantities of selenium or sulfur as inhibitors. In addition, other processes that represent refinements of these methods have also been proposed. One example is the process described in Kokoku No. 13846/1979 for warm rolling steps interspersed between a high-reduction cold rolling process, which is a refinement of the above-described process utilizing an AlN precipitate phase. Another example is the process described in Japanese Laid-open Patent Publication (Kokai) No. 93823/1981, which calls for quenching following intermediate annealing during the final cold rolling step; this is a refinement of the above process in Kokoku No. 14737/1982. The improvements achieved by these methods have allowed magnetic flux density B.sub.10 values of above 1.89 T, and core loss W.sub.17/50 values of below 1.05 W/kg to be achieved, making it possible to obtain high flux density, low core loss product.
The energy crisis several years ago precipitated strong demands for sharp reductions in electrical power loss in transformers and other electrical equipment, and at the same time raised hopes for greater reductions in core loss by iron core materials. One process for manufacturing grain-oriented silicon steel sheet of extremely low core loss that was recently disclosed in Kokoku No. 2252/1982 involves the reduction of core loss by laser irradiation of the surface of a finished sheet utilizing an AlN precipitate phase at intervals of several millimeters and perpendicular to the rolling direction so as to introduce artificial grain boundaries. However, methods introducing such artificial grain boundaries form local areas of high dislocation density. The resulting product remains stable during use only at low temperatures below 350.degree. C., which is a decided disadvantage.
The inventors have conducted research on the mechanisms for the formation and growth of secondary recrystallization grains in the "Goss" orientation of grain-oriented silicon steel sheet, but on the basis of just x-ray diffraction studies have been unable to make any significant progress towards achieving grain-oriented silicon steel sheet with a higher magnetic flux density. As x-ray diffraction was far too inadequate for meaningful studies, they developed a new transmission Kossel apparatus that employs scanning electron images; this was disclosed in Kokai No. 33660/1980 and Japanese Laid-open Utility Model Publication No. 383349/1980. Using this apparatus, we closely studied samples of hot-rolled sheet, intermediate annealed sheet, decarburization and primary recrystallization sheet, initial secondary recrystallization sheet, and other sheet collected during the manufacture of grain-oriented silicon steel sheet. As a result of these studies, they made the following new discoveries (Y. Inokuti, et al., Trans. ISIJ, 23, p. 440, 1983):
(1) Nucleus formation of secondary recrystallization grains with an orientation of {110}&lt;001&gt; arises from strain-free regions of unrecrystallized grains having a {110}&lt;001&gt; orientation near the surface of the heat-rolled sheet and is inherited by means of structure memory.
(2) The secondary recrystallization nuclei having a {110}&lt;001&gt; orientation preferentially formed near the surface of the steel sheet following the decarburization/primary recrystallization annealing step that precedes secondary recrystallization annealing are large nuclei arising from the coalescence of several primary recrystallization nuclei of {110}&lt;001&gt; orientation.
(3) If a small amount of molybdenum is added, this inhibits recrystallization in the vicinity of the hot-rolled sheet surface, resulting in the preferential formation of unrecrystallized grains with a {110}&lt;001&gt; orientation. Moreover, unrecrystallized grains with a {110}&lt;001&gt; orientation that become secondary recrystallization nuclei are preferentially formed. The present occupancy by strain-free regions of unrecrystallization nuclei with a {110}&lt;001&gt; orientation that become secondary recrystallization nuclei is about three times as large as when molybdenum is not added, and the frequency of {110}&lt;001&gt; secondary recrystallization nuclei formation is also about three times as great (Y. Inokuti et al., Tetsu-to-Hagane 69, p. 1284, 1983).
On the basis of these new findings, the inventors also conducted studies on the optimal decarburization and primary recrystallization annealing conditions for grain-oriented silicon steel sheet. In particular, they conducted a series of experiments on the high-grade grain-oriented silicon steel sheet with improved surface properties resulting from the addition of a trace quantity of molybdenum which they proposed in Japanese Patent Application No. 90040/1983 to determine the optimal decarburization and primary recrystallization annealing conditions. When a small amount of molybdenum is added, this has a number of effects: (1) it delays recrystallization, increasing the frequency of secondary recrystallization nuclei formation; and (2) the forsterite following secondary recrystallization annealing forms a uniform thin coating. They conducted a careful study to determine the annealing conditions that maximize the effects of the presence of a small amount of molybdenum, and discovered from this that outstanding grain-oriented silicon steel sheet with a high magnetic flux density and a low core loss can be obtained through decarburization primary recrystallization annealing by rapid heating at an average rate of temperature increase of over 10.degree. C. per second from 400.degree. to 750.degree. C., annealing at 780.degree. to 820.degree. C. for 50 seconds to 10 minutes in an oxidizing atmosphere with a P.sub.H.sbsb.2.sub.O /P.sub.H.sbsb.2 of 0.4 to 0.7, and annealing at 830.degree. to 870.degree. C. for 10 seconds to 5 minutes in an oxidizing atmosphere with a P.sub.H.sbsb.2.sub.O /P.sub.H.sbsb.2 of from 0.008 to 0.4. This led ultimately to the present invention.
The object of the present invention is to provide a process for manufacturing grain-oriented silicon steel sheet with an increased magnetic flux density and very low core loss.